This invention relates to an improved electrostatic recording material, particularly to an electrostatic recording material which comprises an electroconductive transparentized base sheet having a finely embossed surface and a dielectric layer thereon.
The electrostatic recording system has recently been widely applied to a facsimile or a computer input-output system suitable for high speed recording in a high speed telecommunication system, a high speed graphic reproduction system, etc. The electrostatic recording material as a recording medium basically comprises a highly dielectric layer, which serves as an electric-charge-retentive layer, and an electroconductive base sheet which supports the dielectric layer. Electrostatic images of electrical signals formed on the dielectric layer are made visible with a developer comprising a toner and a carrier which has a polarity opposite to the polarity of the electrostatic image charge, and fixed as permanent visible images by further treatment.
The electrostatic recording system does not involve any photoelectric conversion in the process of forming electrostatic latent images on the dielectric layer. The electrostatic recording material of the invention, therefore, does not include any so-called photosensitive element.
There have been proposed various methods for forming electrostatic latent images on a dielectric layer. Among them there are a method of imparting electric signals on the dielectric layer by making it contact with an electroconductive material such as a metal; a method of transforming electrostatic latent images formed on the other medium to the electrostatic recording material, which is known as the "TEST" method and disclosed, for example, in PHOTOGRAPHIC SCIENCE AND ENGINEERING, Vol. 9, No. 1, January-February 1965; and a method of forming discharged patterns on the surface of the recording material with use of a photoconductive film as a switching element, which is known as the "WALCUP" method and disclosed, for example, in U.S. Pat. No. 2,833,930. The electrostatic recording material described can be used in any of those methods for forming electrostatic latent images.
The electrostatic recording system described, however, is distinct from the system for forming latent images by photoelectric conversion with the utilization of a photosensitive material in the recording sheet. In the electrostatic recording system, it is very difficult to obtain multiple reproductions at one time. Therefore, the recorded electrostatic recording material is often used as a master copy for obtaining further multiple copies. In another case, a multiplicity of the electrostatic recording papers recorded with a facsimile or printer plotter are piled and used as a so-called overlay paper. To prepare the electrostatic recording material which is used for those purposes, commercially available tracing papers of glassine papers have been generally used as base sheets. However, such conventional base sheets involve some fatal disadvantages when used for the electrostatic recording material.
To obtain tracing paper or glassine paper having a desired transparency, beating of pulp must be carried out to an utmost extent until cellulose fibers are extremely fibrillated. Consequently, the fibers are injured to impair the paper making property and the physical strength of the paper obtained is unsatisfactory. In addition, since many hydroxyl groups which are sensitive to moisture or water are exposed on the surfaces of the fibers as a result of a high fibrillation, the paper becomes well absorptive of water and accordingly the dimensional stability of the paper is extremely degraded. Owing to those disadvantages, the workabilities for coating and processing in the manufacturing of the electrostatic recording material are significantly lowered. Moreover, the electrostatic recording material produced with use of the above conventional base sheet tends to curl by a change in moisture of the atmosphere or in contact with water and tends to be brittle during the process of fixing treatment with heat. These also lead to make difficult to handle and lower the workability. The above conventional tracing papers and glassine papers further have such a disadvantage that the adsorbed water which is combined with hydroxyl groups is vaporized and expanded to form bubbles in the paper when it is heated, which is the so-called blister phenomenon.
In the electrostatic recording material, a physical or geometrical figure of the surface of the recording material other than the electrical function constitutes an important factor in the recordability. For example, the geometrical uneven surface structure of the dielectric layer has an intimate relationship with a threshold voltage, and a proper roughness of the surface contributes to enhancing a voltage sensitivity and a recordability of the electrostatic recording material. In addition, the geometrical surface structure bears relationships with a writability of the dielectric layer, a fixing ability of toner, a blocking resistance and a slipping property (friction coefficient).
In order to prepare the improved electrostatic recording material, there have been made various attempts, in which almost attention has been given to the improvement of dielectric layer itself. There has never been any attempt to improve the base sheet. For example, there has been proposed a method in which the granular materials (spacer pigment) having a proper diameter are added to the dielectric layer. However, the addition of the particulate materials leads to a degradation of electrical characteristics such as an insulation resistance and a dielectric property at a high humidity, and it makes the process troublesome. Since the electrostatic recording material is partially equivalent to a condenser, an allowable deviation of the thickness of the dielectric layer should be limited to a narrow range. However, the dielectric layer is formed generally with the use of conventional coating devices such as a roll coater, an air knife coater and a blade coater as well as on making a usual coater paper, so it is very difficult to obtain a uniform dielectric layer with a required accuracy. There are two leading factors affecting the accuracy in the above methods. One of them is a fluidity of coating composition, and another is a surface condition of the base sheet. Since the coating composition has a fluidity upon the coating procedure, it is affected by the surface condition of the base sheet and by the kinetic characteristic of coating composition until the coating composition is set on the surface of base sheet. Accordingly, when a paper having a very smooth surfaces land excessive water absorption resistance such as a glassine paper is used as a base sheet for the electrostatic recording material, it is very difficult to form a uniform dielectric layer on it.
According to the invention, the above described problems in the prior arts can be solved. This invention is based on a discovery which is against the heretofore accepted theory that a heavy beating is required for obtaining a desired transparency of the base sheet. This invention is characterized by using the ordinarily or lightly beaten pulp which is used for forming general paper, and by subjeting the resultant paper to heat and pressure with the use of a special embossing metal roll, whereby the disadvantages, i.e. decrease in transparency, inherent in the use of such lightly beaten pulp can be avoided.
An object of the invention is to provide an improved electrostatic recording material having an excellent balance in quality and to provide a process for making it.
Other objects and advantages of the invention will become apparent from the following descriptions.